1. Field of the Invention
This invention relates to the grouting of tubular and support members of offshore structures used in the oil and gas industry.
2. Description of the Prior Art
The search for oil and gas has extended into the deep, rough waters of the outer continental shelf areas. To conduct drilling and production operations in such offshore areas it has become necessary to employ the use of larger and more sophisticated offshore platforms.
Although fixed offshore platforms are of varying sizes and designs, most are installed at their offshore locations by the same general technique. Normally, the platform which is a support structure fabricated onshore, is towed to an offshore location where it is flooded and allowed to settle in an upright position. When uprighted, the legs of the platform will usually settle a short distance into the sea floor. However, it is necessary to give the platform additional support by driving a pile through the platform legs and into the sea floor. The annulus defined by the pile and the platform leg is then filled with cement to form a bond between the leg and the pile. Grouting of the annulus in this manner secures the platform to the sea floor and adds weight to the platform, thereby enhancing its stability in rough waters.
During grouting of the platform legs several problems can arise which would result in faulty or inadequate cementing. The most serious problem is lost circulation which occurs when the weight exerted by the annular column of cement forces the cement into the soft or weakly consolidated formations surrounding the platform leg. With cement escaping into the formation, circulation is lost and the annulus surrounding the platform leg cannot be filled. Another potential problem is improper setting of the cement. Maximum strength and support is given to the platform legs only if the cement is properly cured. Contaminants such as sea water or entrained air can alter the setting properties of the cement resulting in a weakened bond between the pile and platform leg.
In order to detect problems arising during grouting operations it is necessary to monitor the location and setting of the cement. For example, cementing would be halted once the cement reaches a predetermined level within the platform leg that would give the leg adequate support. Failure of the cement to reach certain levels after known volumes had been pumped would be indicative of either lost circulation or a mechanical failure within the platform leg such as packer failure. Likewise, monitoring the set of the cement would indicate whether the cement is curing at a proper rate and whether it is uncontaminated.
With the very tall platforms that are now being used it is becoming increasingly difficult to properly monitor the grouting operation as it is taking place. Manual inspection by divers becomes unfeasible when water depths exceed diver capabilities. Remote visual inspection by subsea vehicles or submerged television cameras is expensive and does not provide any information relating to the setting of the cement. Temperature measuring probes located within the walls of the platform have been used to detect cement location but these probes have not always been reliable because the temperature of the cement tends to rapidly equilibrate with the temperature of the surrounding sea water.
Thus there exists a need in the art for a reliable cement detection and monitoring system which accurately indicates the location and setting of cement used in grouting offshore platform legs.